The present disclosure is directed to improved gage cutting teeth on milled teeth rolling cone rockbits. It has been common to plate over with a hard metal outer layer on a milled steel tooth on a cone. A representative currently used row of teeth, which extend to the gage configuration is shown in FIG. 5 of U.S. Pat. No. 5,131,480. The above mentioned patent describes certain of the difficulties in cutting to gage diameter with a broad, flat, gage positioned surface. Drawbacks of this sort have been acknowledged recently in a Hughes Christensen GT Technology Bulletin (date unspecified). The drawbacks in that bulletin of Hughes are directed primarily to tungsten carbide inserts (TCI hereinafter) in a rockbit.
While that article is directed to TCI technology, the problem remains substantially the same for a cone in a drill bit which is made with milled teeth. At bottom, the problem simply is that a large flat area on the gage cutting surface cuts inefficiently. One attempt to solve that is set forth in U.S. Pat. No. 5,131,480. That reference teaches the advent of a partial covering of hard metal. It is located at the normal gage cutting surface. It is clad to the milled teeth of the cutter. By placing it at that location, this reduced the contact area existing between the rolling cone cutter and the surrounding borehole wall. FIG. 4 of the ""480 patent shows a partial coating of hard metal material. This approach reduces the area of contact between the tooth as a whole and the borehole wall. It is, however, mechanically limited in that one edge of the hard cutting surface is exposed and therefore not supported. If the area of the partial hard facing is made narrow and therefore sharper, the hard facing material has a reduced bonding area holding it to the milled tooth and it is more likely to break off as a result of a planar stress fracture which chips off that piece of coating.
By contrast, the present disclosure sets forth a novel design for a gage tooth of the milled tooth variety. This is a structure which cuts better because it has a reduced or minimum contact area with the borehole wall. Yet, the extra hard metal surface is made stronger and bonded better because greater strength is achieved in the connective bond (a planar area). The gage facing surface of each tooth for the whole row of teeth on the cutter, is shaped so that only a narrow surface of each tooth confronts and thereby contacts the borehole wall. Adjacent areas on each tooth slope away from the wall. The narrow gage facing surfaces, these sloping areas and other areas are commonly covered with a suitable hard metal facing material. By providing a complete coverage over the gage facing areas, strength of the hard facing material is increased. Bonding of the hard facing material to the underlying milled tooth body is improved. The narrow cutting surfaces are more efficient and are less likely to fail. The narrow cutting edge concentrates the cutting force through a more narrow area and resulting in more efficient cutting. More efficient cutting in the gage area helps increase the overall rate of penetration resulting in lower cost for each foot drilled. More efficient cutting in the gage area can also result in an increase of total depth drilled reducing costs even more.
More efficient cutting in the gage area also makes a bit more suitable for directional drilling on mud motors.